1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to an improved twin well forming method for a semiconductor device which prevents an uneven surface of a semiconductor substrate during a twin well formation, thereby enhancing a product reliability.
2. Description of the Background Art
FIGS. 1A through 1H are cross-sectional views sequentially illustrating a twin well forming method for a semiconductor device according to the conventional art.
As shown in FIG. 1A, on a semiconductor substrate 1 there is formed a first silicon oxide layer 2. A silicon nitride layer 3 is formed on the first silicon oxide layer 2.
In FIG. 1B, a photoresist pattern 4 is formed on the silicon nitride layer 3. Using the photoresist pattern 4 as a mask, the silicon nitride layer 3 is etched to form a silicon nitride layer pattern 3a. At this time, an upper surface portion of the first silicon oxide layer 2 is externally exposed through the etched hole.
Then, in FIG. 1C, phosphorus ions 5 are implanted through the exposed first silicon oxide layer 2 into the semiconductor substrate 1.
As illustrated in FIG. 1D, the exposed first silicon oxide layer 2 is selectively oxidized at a high temperature to obtain a second silicon oxide layer 6. At this time, the phosphorous ions 5 implanted into the semiconductor substrate 1 drive-in downwardly in the semiconductor substrate 1.
Referring to FIG. 1E, the silicon nitride layer pattern 3a is removed by an etching.
Next, as shown in FIG. 1F, using the second silicon oxide layer 6 as a mask, boron ions 7 are implanted through the first silicon oxide layer 2 into the semiconductor substrate 1.
As shown in FIG. 1G, the semiconductor substrate 1 is annealed to drive-in the boron ions 7 downwardly in the semiconductor substrate 1. At this time, the phosphorous ions 5 are further diffused downwardly in the semiconductor substrate 1.
Finally, as shown in FIG. 1H, the first silicon oxide layer 2 and the second silicon oxide layer 6 are removed from the semiconductor substrate 1 to realize the conventional twin well formation for a semiconductor device.
However, the conventional twin well forming method for a semiconductor substrate causes an uneven, stepped surface on the semiconductor substrate, thereby deteriorating a reliability of the semiconductor device.